Electrical contact systems having spring loaded, movable contacts are employed in circuit board and electronic circuit test systems and the like in which electrical contact is made between a circuit or device and one or more test points. The electrical contact system generally comprises a metal barrel having a contact plunger telescopically slidable therein and a spring disposed within the barrel for biasing the contact plunger to a normally outward position. The contact plunger has an outer end provided with a probe tip of a selected configuration to engage a test pad or the like of a circuit board or other device. As test systems employ higher radio frequencies in testing procedures there is an increasing need to provide the shortest possible so-called electrical path in a matched impedance system. Contact assemblies have moving parts and inherently have a degree of mismatch within the manufacturing tolerances and materials employed. Any mismatches which do occur are directly related to the length of the contact assemblies so that the longer the assembly, the greater the mismatch. In copending U.S. application Ser. No. 09/145,914, now Pat. No. 6,053,777 which issued Apr. 25, 2000 and is assigned to the assignee of the present invention, the subject matter of which is incorporated herein by this reference, an electric contact assembly having an improved short length is described accomplished, inter alia, by attaching the ground plunger to the coaxial cable. The system has an inner contact assembly having an electrically conductive inner plunger barrel received on the signal conductor of the coaxial cable and spaced from the ground plunger in coaxial relationship therewith. An inner contact portion of an inner plunger of the contact assembly is received in the plunger barrel and biased by a first spring to extend a selected amount in a given direction beyond the ground plunger when in at rest position and a second spring biasing the ground plunger and coaxial cable in the given direction with the second spring having a spring force greater than the first spring. In operation, the inner contact portion which extends outwardly beyond the ground plunger and outer contact portion will engage the test site and be forced inwardly until it is coplanar with the outer contact portion of the ground plunger. Both contact portions, as well as the coaxial cable, will then move inwardly together for another selected distance to ensure that predicatable contact pressure is obtained for both the inner and outer contact portions. By combining the motion of the inner and the outer contact portions, the overall length of the assembly is shortened thereby minimizing any impedance mismatch. However in certain applications, it is preferred to mount the coaxial cable so that it is immovable relative to the test fixture.
The ground plunger and the inner contact assembly of the electrical assembly described in Ser. No. 09/145,914 are maintained in concentric, coaxial relation with one another using first and second spaced apart annular spacer members of dielectric material so that the effective dielectric therebetween approximates that of air. In a modified embodiment, the spacer members are formed into an integral structure with a reduced diameter intermediate web joining two spacer portions, the reduced diameter web providing an open annular space with a resulting near air dielectric.